An array antenna presents the specific feature of having an aperture made up of a large number of radiating elements; the radiation from the antenna being synthesized from the radiation of each radiating element. Such antennas are a recent development and they are currently to be found in applications to a wide variety of fields, such as:
air traffic control; PA1 satellite reception (television, message transmission, and communication with mobiles); and PA1 space antennas: remote sensing and observation of the Earth (radars), data relays, and telecommunications antennas.
The frequencies covered range from VHF and UHF up to millimetric wave frequencies. When the radiating elements are controlled individually in amplitude and/or in phase, the antenna is said to be an active antenna. It is possible to chose the shape of the radiation pattern of the antenna, for example, so as to select widely different coverage regions (shaped beam, wide beam, or narrow beam) or so as to perform electronic scanning.
By their intrinsic radio performance level, their suitability for being put into arrays, and the technology used to make them, the radiating elements which form an antenna dictate its ultimate performance, its cost, and its technical characteristics (mass, reliability, and resistance to the environment).
Since an antenna is made up of from a few tens to a few thousands of such radiating elements, the unit cost thereof is a determining factor in the overall cost of the antenna. The same type of reasoning also applies to other parameters such as mass. The choice of technology is important because it makes it possible to simplify problems of matching the antenna to its environment. For example, for space applications in geostationary orbit, it is important to be able to control antenna temperature by simple means (thermal coverings, paints), without calling for heater power which would spoil the energy budget of the system. Under such conditions, temperature ranges as great as -150.degree. C. to +120.degree. C. may arise, given the thermo-optical characteristics of the surfaces. Such an antenna is further subjected to fluxes of charged particles that must neither damage the materials, nor cause electrostatic discharge after accumulating on insulating regions or on regions that are poorly grounded.
An antenna must retain all of its radio qualities even after having been subjected to high mechanical stresses during launching.
Some of these qualities, e.g. the ability to generate only very low levels of passive intermodulation products, are extremely closely linked to the technologies used (the association of the various materials, and the geometry of the elements), and to the way in which they withstand the operating environment (in particular the thermal environment).